This invention relates to a proximity detector, and especially to a ferrous-gear-tooth Hall-transducer, or other magnetic-field-to-voltage transducer, capable of detecting the leading and trailing gear tooth edges of an adjacent rotating ferrous gear, or other magnetic articles, and more particularly relates to such a Hall sensor with detection thresholds that adapt to the peak to peak amplitude of the Hall voltage.
The term "magnetic" as used herein applies to magnetized bodies, ferrous bodies and other bodies having a low magnetic reluctance that tend to alter the ambient magnetic field.
In U.S. Pat. No. 5,442,283, issued Aug. 15, 1995 there is described a Hall-voltage slope-activated sensor capable of detecting the rising and falling edges of a gear tooth, which sensor includes a circuit for tracking a slope of a Hall voltage and briefly holding the ensuing peak voltage before producing a pulse signal indicating the onset of the following Hall-voltage slope of opposite direction. The Hall voltage holding circuit includes a capacitor and circuit means for controllably leaking charge out of or into the capacitor for preventing false tripping of a comparator that provides the pulse output signal. The holding voltage of the capacitor thus has a droop which leads to increasing loss of holding accuracy as the speed of gear tooth passage becomes slower, and therefore the detector has a minimum gear tooth speed at which accurate detection is possible.
The changes in the ambient magnetic field and corresponding changes in the transducer voltage caused by the passing of magnetic articles tend to vary. Most such proximity detectors of the prior art produce a high binary output voltage indicating proximity of a passing article, and produce a low binary voltage when the article recedes from the detector.
The transition in detector output voltage form low to high typically is triggered by a comparator that determines when the transducer voltage rises to equal a fixed internal threshold voltage reference, or in the case of the above described slope-activated sensor, determines when a transducer voltage peak has just occurred and the signal voltage drops a predetermined incremental voltage from the peak value.
These prior art proximity detectors, having fixed threshold voltages, produce a low to high (or high to low) output voltage that corresponds to different locations in the transducer voltage waveform when there are changes in the amplitude of the transducer voltage.
The sources of such changes in transducer voltage amplitude are many. For example, gear teeth (articles) may have different ferromagnetic properties from tooth to tooth and undulating changes in the spacings (air gap) gear teeth to detector caused by eccentricity of the gear. Also, changes in temperature can cause changes in air gap dimensions and in the sensitivity of the transducer and transducer-voltage amplifier. Furthermore, the magnetic-field-to voltage transducer in a proximity detector typically includes an internal DC offset voltage that varies with mechanical stresses and temperature variations.
Such changes in the transducer voltage therefore cause shifts in the timing of proximity detection relative to the actual distances of article approach and receding at which these transducer voltages exceed or fall below the fixed thresholds. This results in loss of accuracy in proximity detection that has become less and less tolerable especially when employed for detection of the rotational position of a gear by sensing the proximity of the gear teeth.
It is an object of this invention to provide a proximity sensor that generates a binary output voltage wherein the transitions accurately correspond to a definite point of approach and a definite point of receding of a passing magnetic article.
It is a further object of this invention to provide a magnetic article proximity detector that periodically determines when the amplitude of the magnetic-field-to-voltage transducer voltage has changed significantly, and adjusts the detection threshold as needed to be essentially a predetermined constant percentage of the peak to peak value of a changing detector-transducer-voltage amplitude.